It is well known in the petroleum industry that, at the end of primary production of oil from a subterranean formation, secondary and tertiary recovery methods may be used. Primary production refers to the production of oil by simply flowing oil (possibly in mixture with water, gas or both) from the formation through a suitable tubing in a well under formation pressure or pumping the oil from the formation. In primary production, the oil flows from the formation into the wellbore for recovery under formation pressure. Secondary and tertiary recovery relate to the use of various techniques for causing the oil to flow from the formation into the well at a suitable rate. Such techniques involve water flooding, gas flooding, miscible gas flooding and the like, as well known to those skilled in the art. In general, such techniques involve the passage of a substance such as water, steam, carbon dioxide, light hydrocarbon gases containing from 1 to 5 carbon atoms, nitrogen, mixtures thereof and the like, from a first injection well or wells through the subterranean formation to a production well or wells to cause the oil to move into the production well or wells for recovery, generally with the injected fluid.
A problem in such processes is the existence or development of high permeability zones in the formations between the injection wells and the production wells through which the bulk of the injected material flows. Initially, this flow pushes the oil found in the high permeability zones to the recovery wells. Subsequently, the injected material simply bypasses the less permeable portions of the formation and flows directly to the recovery wells through the high permeability zones. In such operations, it is desirable to plug or otherwise restrict the high permeability zones. The restriction of flow through the high permeability zones results in causing the injected fluid to pass through the lower permeability zones, and to push oil from those zones the recovery wells.
One technique used to block the high permeability zones is the use of gelable compositions which are injected into the formation and allowed to set in the high permeability zones. One suitable gelable composition which is widely used is an aqueous solution of polyacrylamide. The polyacrylamides are caused to gel by mixture with a multivalent metal cation such as trivalent chrome. It is well known in the art that such mixtures must be retarded to prevent premature gellation of the polyacrylamide with the multivalent metal cation at typical oil reservoir temperatures. Other polyvalent ions may also be used but trivalent chrome is a widely used multivalent cation.
Previously, it has been considered necessary to produce chrome ligands for use in combination with a second ligand as disclosed, for instance, in U.S. Pat. No. 5,338,465, "Aqueous Gelable Composition With Delayed Gelling Time", issued Aug. 16, 1994 to Thomas P. Lockhart and Paola Albonico. The use of these materials requires that the first organic ligand be prepared and transported to the well, and that the second ligand be transported to the well. These materials are frequently solutions and their transportation to wells in remote locations can constitute a substantial expense.
Since it is necessary that the gellation of the polyacrylamides with multivalent cations be inhibited to permit the injection of the getable composition into the zone of interest, alternative methods have been sought for accomplishing this objective without the use of the relatively expensive, and more difficultly handled, ligand solutions.